The present invention relates to hydrophones, and more particularly to hydrophones employing piezoelectrically active elements of the polymer membrane type. The hydrophone is primarily intended for use in high pressure amplitude acoustic shock wave field measurements
There is a need for precise, quantitative measurement of high amplitude (10.sup.8 Pa) acoustic pressure distributions or shock wave fields which are present, for example, in the focal region of lithotripters. Such lithotripters use focused ultrasonic shock waves to shatter concretions such as kidney stones in the kidney of a patient. Quantification of these shock wave fields is necessary for determining the safety and performance characteristics of these hydrophones.
These shock wave fields possess very steep shock wave fronts with rise times well below 1 .mu.s and, in some instances, very small focal volumes (the region in which the pressure is greater than one half the maximum pressure). The hydrophones, or acoustic sensors, used to measure these fields must therefore possess both a broad bandwidth (up to 100 MHz) and fine spatial sensitivity (less than 1 mm active region) in order to accurately quantify the pressure levels. The bandwidth requirement drives the design toward the use of thin
(less than 25 .mu.m), acoustically transparent films of piezoelectric polymer material, such as polyvinylidene difluoride. The electrode material used in the active region of the hydrophone must also be in the form of a thin layer, in order to prevent acoustically loading the material or altering its bandwidth characteristics. The spatial sensitivity requirement results in the use of small active apertures referred to as sensitive elements with thin connecting leads. This combination creates a hydrophone which is susceptible to damage from the action of the shock waves. The electrode material is slowly removed under shock wave action, i.e., cavitation, until the electrical connection from the sensitive element to the recording instrumentation is rendered unreliable. Thus, the primary difficulty with currently available hydrophones is the unreliability of the measurements from the sensitive element over time. Since each shock wave causes a slight change in sensitivity, it is difficult to predict whether the results of any shock wave measurement (except the first) will be valid.
The prior art hydrophones have been based on a membrane-type design, such as that discussed in U.S. Pat. No. 4,433,400. As noted above, this type of design is susceptible to damage from shock wave action. The difficulty in using this membrane-type design in a shock wave environment was addressed in U.S. Pat. No. 4,734,611, which disclosed a design which used extra membranes with conductive coatings to conduct the electrical signal from the sensitive element. These extra membranes can interfere with the acoustical properties of the hydrophone, and result in a more complicated physical construction. Thus, neither prior art design is completely adequate for calibrated measurements of high pressure lithotripter shock wave fields, either due to problems of fragility and stability of the sensitive element, or due to the complexity of the physical construction.
Another design for a hydrophone which was described as being appropriate for shock wave measurements was a needle-type design disclosed by Platte in "A Polyvinylidene Fluoride Needle Hydrophone for Ultrasonic Applications," 23 Ultrasonics at 113-18 (May 1985) and by Lewin in "Miniature Piezoelectric Polymer Ultrasonic Hydrophone Probes," 19 Ultrasonics at 213-16 (May 1981). While the needle-type design does survive better than the membrane design, it is also eventually destroyed by shock wave action. In addition, the needle-type hydrophone does not faithfully reproduce the complete acoustic pressure wave form.
Other designs for hydrophones which have been described as being useful in lithotripsy fields include that disclosed in U.S. Pat. No. 4,803,671 which is a design substantially similar to that described in U.S. Pat. No. 4,653,036 (described above). The design in the '671 patent uses a double membrane around the piezoelectric polymer membrane to provide constant liquid immersion of the sensitive element, regardless of the surrounding fluid. The double membrane design does not, however, address the problem of sensitive element destruction by shock wave action. The sensitive element design disclosed in U.S. Pat. No. 4,813,415 does not produce a pressure versus time wave form, but is merely a shaped, thin foil sensitive element which is subjected to shock waves and then optically inspected for damage. The location, diameter, depth, profile, and volume of the deformations in the foil provide information on the focusing and intensity of the shock wave.
Finally, U.S. Pat. No. 4,764,905 describes a spherically shaped piezopolymer membrane design for a hydrophone which matches the presumed wave front from a spherically focused shock wave generator. The spherically shaped design is only appropriate for spherically focused systems, and completely integrates the acoustic pressure wave form without any spatial resolution. This spherically shaped design again does not address the problem of sensitivity changes in the sensitive element caused by shock wave action on the polymer material of the electrodes.
Recently, Everbach described in "An Inexpensive Wide-Bandwidth Hydrophone for Lithotripsy Research," 87 J. Acoustical Society of America, at S128 (1990), the design of a membrane hydrophone with a disposable sensitive element. This design has application for shock wave measurements and claims certain attributes such as reproducible sensitivity of measurements without calibration, a disposable sensitive element, a compensating preamplifier, and a signal-limiting circuit. The design does not, however, address the problem of when to replace the sensitive element, that is, when it has been sufficiently damaged to require replacement. This design does not address the possibility of establishing an exact limit on the number of shock waves that each hydrophone may sustain without damage, since this will depend upon the intensity of the shock wave, the position of the hydrophone in the shock wave field, the conditions of the liquid used to couple the shock waves to the hydrophone and other factors. This design again leaves the operator in doubt as to the validity of the measurement results.